480 keport— 1884. 



Thomson, that whereas copper is negative with respect to iron in air, it 

 is positive with respect to iron in H 2 S. It would appear therefore that 

 the reason why the results of metals by the ordinary " contact force " ex- 

 periments harmonise so well with the comparison by dipping both metals 

 in water or an oxidising electrolyte, is not because the E.M.F. between a 

 metal and a gas or an electrolyte is small, but because the properties of 

 air agree to a certain extent with those of ordinary electrolytes. For 

 if the active component of the electrolyte is sulphur the results are quite 

 different, and the same kind of difference occurs when hydrogen sulphide 

 is substituted for air.' 



In their third paper, 1 therefore, we find Ayrton and Perry's views 

 changed, and they clearly state that their experiments, like all those based 

 on inductive methods, leave the question of air contacts quite undecided. 



They then go on to say: 'One way of determining the E.M.F. of 

 contact in volts between a substance and air, and a way we hope shortly 

 to employ, is to repeat all these exact contact experiments in different 

 gases [it is not quite true that this would give the results required, 

 because it would only give differential effects ; very important to observe, 

 no doubt, but not the same as observing the actual contact force of air or 

 of gas]. . . . We shall thus ascertain whether the contact difference of 

 potentials of a substance and a gas differs much for different gases. 

 Qualitative experiments in this direction have already been made with 

 very interesting results by Mr. Brown, of Belfast, but his experiments 

 differ from ours in not being quantitative.' [Only, as their experiments 

 do not yet exist, Mr. Brown's have still some value.] 



In vol. xii. of ' Wiedemann's Annalen ' 2 I find some interesting experi- 

 ments by Schultze-Berge on contact force between metals and gases. He 

 uses a condenser and different gases, but the plates of his condenser are 

 both of the same metal, and he coats one of the. plates with a film of the 

 gas, say chlorine, or hydrogen, or ozone, and leaves the other covered with 

 air. To measure the potential difference he employs the compensation 

 method of Pellat, and his arrangement seems fairly satisfactory. But he 

 does not explicitly enter on the question as to the seat of E.M.F. in the 

 Volta experiment (except in a controversy with Professor Exner). He 

 assumes that a contact force between metals and metals and between 

 metals and liquids has been established, and he wishes to extend it to the 

 contact of a metal and a gas. Believing firmly in the existence of films 

 of condensed gas at the surface of a solid, which films require time for 

 their formation or removal, he deems it sufficient to soak one of the plates 

 of his condenser in the gas to be examined, and then to take it out and 

 measure the difference of potential between it and the other plate coated 

 only with a film of air. Tried thus, ozone rendered gold, platinum, and 

 brass negative as against the corresponding air-covered plate. Hydrogen 

 rendered its platinum sti*ongly positive, but its influence on gold was 

 slight, and on brass uncertain. Chlorine and bromine made platinum 

 negative, and ammonia made brass positive. 



It may be readily objected that what the soaking with gas accomplished 

 was the formation of not only a film of gas, but a film of actual chloride, 

 oxide, or other combination. Against this is to be urged the fact that 

 after removal from the gas the effect diminished with time, and the plates 



1 Phil. Trans., 1880. 



2 Schultze-Berge : ' Ueber die Elektricitatserregung beim Contact von Metallen 

 und Gasen.' Ann, dcr Phys. u. Chem., xii. 293, 1881. 



